In U.S. Pat. No. 5,321,718, Waarts et al. describe a coherent light source having an astigmatism-correcting lens system positioned in the path of a high power, but astigmatic, coherent light beam from a semiconductor optical source such as a flared-amplifier-type MOPA device or a flared-resonator-type laser diode. A number of lens configurations are described, which include combinations of cylindrical and spherical lens surfaces. While most of the embodiments use from two to four lenses, one embodiment employs a single lens with two crossed positive cylinder lens surfaces. Another embodiment uses a lens having a positive toric surface and a planar surface. All of the lens systems are adapted to provide a modified astigmatism-free light beam from the astigmatic light received from the semiconductor optical source. The astigmatism-free light is useful for many laser applications, including frequency conversion, of which a number of configurations are disclosed.
In U.S. Pat. No. 5,369,661, Yamaguchi et al. disclose an optical system for coupling light from a semiconductor laser array into a solid-state laser medium or into an optical fiber. The optics include a gradient index (GRIN) lens array to condense the individual light beams emitted with a large divergence angle from the semiconductor laser array to form parallel collimated light beams. A separate aspherical lens then converges the light beams into a single beam spot. Stacks of two or more laser arrays with corresponding stacks of two or more GRIN lens arrays are also disclosed, which form a 2-D array of parallel light beams. An aspherical lens then condenses the array of light beams to a beam spot for coupling to a fiber. Plural sets of stacked arrays may be combined by arranging their respective optical fibers to form a fiber bundle.
U.S. Pat. No. 5,229,883 to Jackson et al., U.S. Pat. No. 5,081,639 to Snyder et al., and U.S. Pat. No. 5,293,269 to Burkhart et al. disclose lens optics for collimating the diverging light output from diode lasers and diode laser arrays. Jackson et al. use a first cylindrical lens for collimating the light in the fast axis (or transverse direction) and a second binary diffractive optical element or array of such elements, simulating one or more aspheric lens surfaces, for collimating the light in the slow axis (or lateral direction). Snyder et al. use a cylindrical lens having an elliptical or hyperbolic cross-section, while Burkhart et al. use a lens with a circular-cylindrical back surface and an acircular-cylindrical front surface. Both of these cylindrical lenses are formed by means of a fiber lens drawing process from a master or preform having the desired cross-section.
In U.S. Pat. No. 5,216,687, Fujino et al. employ a spherical first lens or a GRIN lens array for collimating the light from a semiconductor laser or laser array in the fast axis, and a bicylindrical second lens with crossed (orthogonally oriented) cylindrical surfaces for focusing the light in both its fast and slow axes to a spot.
In providing lens optics for semiconductor laser sources that emit highly astigmatic light beams, such as flared-resonator-type laser diodes or flared-amplifier-type MOPAs, it is desirable that the optics not only correct for the astigmatism in the light, but also be compact, have a minimum number of refracting surfaces within the constraints of manufacturability, be easily positioned in front of the laser source at the proper locations within the design tolerances, and preferably be inexpensive to make. A minimum loss of brightness is preferred, so that numerical aperture is an important design parameter. Likewise, when arrays of such astigmatic laser sources are used, the corresponding lens arrays need to provide a precise center-to-center spacing between lenslets and be designed, if possible, for maximum beam filling of the emitted array of light beams. Unfortunately, many of these requirements conflict so that trade-offs must be made. A theoretical design calculated from purely optical considerations may include lens surfaces which are difficult and very expensive to manufacture. If the design is limited to easily manufactured lenses with circular-cylindrical and spherical lens surfaces, multiple lenses are required, which must be precisely positioned, and which generally limit the numerical aperture and beam filling factor that are achievable, thus reducing brightness.
An object of the invention is to provide a coherent light source in which the astigmatism-correcting lens optics for high power semiconductor laser sources that emit astigmatic light beams preserve the brightness of the emitted light, while being compact, inexpensively manufacturable and easily positioned for astigmatism-correction and beam collimation. Another object of the invention is to provide an astigmatism-correcting lens array for a diode laser array that is inexpensive to manufacture with maximum beam filling and brightness conservation of the array of emitted beams.